This invention relates to an apparatus and process for detecting an absolute position of an element, and more particularly to an apparatus and process for determining an absolute value by use of signals from a plurality of detectors.
In the prior art, for example, as disclosed in Japanese Kokai Tokkyo Koho No. 53-53350, a detector detecting an absolute position of an element on a machine tool, obtains an absolute value as follows. First, the number of rotations of the element in a direction of a coordinate axis, i.e. the X axis, is derived from a driving unit for the X axis direction and that number is supplied to a reduction train with two (2) stages or three (3) stages. A rotary detector is provided on a rotatable axle of each reduction stage so that a value within one revolution of the rotary detector is read out to obtain an absolute value from the combination of values detected by the rotary detectors. The combination of values above is performed as follows.
Suppose that a table of a machine tool is moved in the X axis direction. The number of revolutions derived from the X axis driving unit is reduced to 1/10 between the first axis and second axis of an X axis absolute position detecting unit. Further, the reduced number of revolutions is again reduced to 1/10 between the second axis and third axis thereof. Furthermore, the reduced number of revolutions is reduced to 1/10 between the third axis and fourth axis thereof. In this instance, the fourth axis rotates less than one revolution over all the length of all the measuring range in the X axis direction.
For example, suppose that the first axis rotates one revolution per 2 mm which is the movement of the table in the X axis direction. The movement of the table corresponding to each revolution of the fourth axis is as follows. EQU 2.times.10.times.10.times.10=2000 (mm)
Therefore, the effective detecting range is 2 meters.
One revolution of the third axis corresponds to 200 mm in the movement of the table while one revolution of the second axis corresponds to 20 mm in the movement of the table.
Thus, in this case, an absolute value within 2000 mm of the table movement can be calculated from the sum of each value of the rotating angles within one revolution of the first, second, third and fourth axes.
However, the disadvantages concerning the above are:
(a) The reduction train becomes larger in size and its inertial moment increases as the effective measuring range is enlarged.
(b) The weighting factor for each value of the axes are different from each other. Thus, errors over one graduated scale on the fourth axis will be 1000 times greater in the first stage.
Accordingly, mechanical accuracy must always be maintained at a high degree even if vibration or wear occurrs in the machine tool during operation thereof.